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CAPE CANAVERAL, Fla. - NASA will start the launch countdown for space shuttle Endeavour's STS-118 mission at 9 p.m. EDT Saturday, Aug. 4, at the T-43 hour point. The countdown includes 27 hours, 3 minutes of built-in hold time leading to a preferred launch time at approximately 7:02 p.m. EDT Tuesday, Aug. 7. The launch window extends an additional five minutes.

During the 11-day mission to the International Space Station, Endeavour's crew will add another truss segment to the expanding station, install a new gyroscope and add a spare parts platform. The flight will have at least three spacewalks. It also will debut a new system that enables docked shuttles to draw electrical power from the station to extend visits to the outpost.

A detailed list of launch countdown milestones and times is available at:

This mission is the 119th space shuttle flight, the 20th flight for Endeavour and the 22nd U.S. flight to the International Space Station. The mission will be Endeavour's first flight in more than four years. The shuttle has undergone extensive modifications, including the addition of safety upgrades already added to shuttles Discovery and Atlantis.

For more information about the STS-118 crew and mission to the space station, visit:

Final preparations of the shuttle's three main engines for main propellant tanking and flight (5 a.m.) Begin filling pad sound suppression system water tank (7 a.m.) Pad sound suppression system water tank filling complete (10 a.m.) Close out the tail service masts on the mobile launcher platform Begin star tracker functional checks (12:50 p.m.)